The colorimetric assay of an analyte from a fluid sample by utilizing a reagent that includes a tetrazolium salt is well known. Schematically, such assay is exemplified as shown immediately below (exemplified for glucose assay): ##STR1##
In the above example, glucose is catalytically oxidized by glucose dehydrogenase (GDH) to gluconic acid. As glucose is oxidized, a mediator, nicotinamide adenine dinucleotide (NAD) is reduced (NADH). A second enzyme, diaphorase, reoxidizes NADH to NAD and reduces the tetrazolium salt to a colored formazan. Alternatively, the enzyme involved in catalytic oxidation of glucose may be glucose oxidase (GOD), and the mediator may be phenazine ethosulfate (PES) (see schematic above). (When GOD is the enzyme involved in glucose oxidation, the second enzyme, diaphorase, is not required for the assay. PES (reduced form) will reduce the tetrazolium salt to the colored formazan, thereby reoxidizing PES.)
The use of tetrazolium salts in a reagent for the assay of an analyte in a fluid sample is disclosed in Baker, U.S. Pat. No. 4,645,742, issued Feb. 24, 1987, Hidehiko et al., U.S. Pat. No. 4,427,771, issued Jan. 24, 1984, and Case et al., U.S. Pat. No. 4,247,633, issued Jan. 27, 1981.
Koever et al., U.S. Pat. No. 4,743,559, issued May 10, 1988, discloses the incorporation of iodate in a reagent test strip that includes a tetrazolium salt. Iodate is added to oxidize ascorbic acid and remove it as an interferrent in assays that involve oxidation-reduction reactions.
Pawlak, U.S. Pat. No. 4,892,817, issued Jan. 9, 1990, discloses moderate stabilization of tetrazolium salt solutions by addition of potassium permanganate, and manganese dioxide. Pawlak does not provide any reagent formulations that include these oxidizing agents, but merely lists in Table 2 of Pawlak that moderate stabilization is achieved in a few cases.
None of these references discloses a reagent that includes a tetrazolium salt and further includes the oxidizing agents (disclosed below ), which are useful for stabilizing the reagent, thereby decreasing the blank reaction observed in assays utilizing reagents that include a tetrazolium salt.